1. Field of the Invention
This invention is related in general to high-surface-area electrodes for capacitor applications. In particular, the invention relates to valve metal electrodes produced by vapor-phase deposition on various substrates in a high-surface-area dendritic form. Substrates include thin valve-metal foils, valve-metal screens, fibers, and polymer films. A multilayer high-speed electrode formation process is used that results in high-surface-area electrodes with high capacitance per unit volume of coating. Such electrodes are anodized inline to form a metal oxide dielectric, a liquid or polymer electrolyte is added, and they are then packed to form a capacitor for various electrical and electronic applications.
2. Description of the Related Art
Aluminum and other valve-metal electrolytic capacitors are usually produced by etching a metal foil to produce a high-surface area and then by anodizing the etched foil to create a metal-oxide dielectric. For a given dielectric thickness, the etching process and the thickness of the aluminum foil determine the resulting capacitance per unit volume. An alternative method is by depositing a high-surface-area porous valve-metal coating on a valve-metal foil and then processing the coated foil much like an etched foil.
When depositing thin films and in particular polycrystalline metal films on a substrate, the film growth and the resulting crystalline structure can be separated into different growth stages. The first phase is a crystal-nucleation stage, where atoms arriving on the substrate are reactively trapped by potential wells or condense on the substrate to form small individual crystals. During this stage the deposited atoms diffuse and bounce on the substrate surface until they condense or react with the substrate. The next growth phase is a crystal-growth stage, during which the deposited atoms grow crystals on the nucleated sites. The next stage represents grain growth by coalescence and by abnormal grain growth due to grain migration. The final growth phase as the thickness of the film increases is a segregation phase induced by process conditions, impurities and additives, all of which produces clearly distinct phases normally referred to in the art as dentritic structures. Because of their large surface area per unit volume, such structures represent the ultimate goal of crystal growth for electrolytic capacitor applications.
When depositing valve metals such as aluminum, the formation of dentritic structures is promoted by high temperatures and the presence of impurities; otherwise, the metal layer does not acquire the large-area configuration necessary for useful capacitor applications. Different dimensional segregates can be formed by varying the temperature of the substrate and a model for substrate temperature dependence proposed by B. A. Movchan and A. V. Demchishin (1969) describes three phases of segregates. A columnar highly segregated structure formed at temperatures below 0.2 Tm(where Tm is the melting temperature of the bulk deposited material), followed by a second phase at temperatures between 0.2 Tm and 0.5 Tm where the grain growth forces the columns to coalesce, and another higher temperature zone where additional grain growth opens the grain boundaries further resulting in a larger fully coalesced crystal structure. A transition zone between 0.1 Tm and 0.2 Tm, added to the model by Thornton Grovenor at al. and Messier et al., also takes into account ambient pressure. Impurities and additives are two parameters that also have a significant effect on the structure of a coating. In the case of aluminum coatings that are of particular interest to this invention, Barna and Adamik have shown that an impurity such as oxygen may be used to disrupt the aluminum atom diffusion and coalescence, leading to disruption of grain boundary growth, resulting in a rough surface coating. The degree of disruption of the normal growth is shown to depend on the ratio of impurity molecules to depositing molecules on the surface of the growing film. Such coatings become increasingly porous as they get thicker, leading to a high surface-area aluminum layer. Therefore, one can use such micro-structured aluminum (or other valve metal coatings) to produce high surface-area capacitors by anodizing the coating to form an aluminum oxide and then introducing a liquid or polymer electrolyte with a counter electrode.
U.S. Pat. No. 4,309,810 teaches a method for producing such micro-structured aluminum coatings for use in electrolytic capacitor applications by evaporating aluminum in the presence of an oxygen impurity. U.S. Pat. No. 5,431,971 and U.S. Pat. No. 5,482,743 teach a similar process on a plate or a roll of aluminum foil, using various mixtures of oxygen that result in a micro-structured deposit that has certain levels of atomic aluminum metal and oxygen. U.S. Pat. No. 7,404,887 and U.S. Pat. No. 7,709,082 teach a similar coating structure for a valve metal such as aluminum using oxygen as impurity, referring to it as a cauliflower and fractal structure. A series of Japanese patents, the latest of which is PAJ245066, teaches a similar structure with defined Al/O2 ratios, referring to it as a columnar structure or fine particles; and U.S. Patent Publication No. 2011/0002088, related to PAJ 288296, teaches a similar aluminum coating structure with defined Al/O2 ratios, referring to the structure as frost pillars and aquatic plants. Other descriptions of this aluminum dendritic growth include tree structures, porous layers, and grain agglomerates.
In all prior art cases what is described is a coating of micro-structured or nanostructured dendritic aluminum coating that has a thickness of a few microns to several tens of microns. Given the thickness of the aluminum coating, during the coating process the temperature of the substrate is necessarily elevated to temperatures in the order of 300° C. or higher, which are in the order of 0.5 Tm or higher for aluminum. Based on the three-zone growth model, at such temperatures the structure of the coating is a dense one, void of fractal, dendritic or porous character. In order to induce a certain level of porosity, Barna and Adamik show that an impurity such as oxygen (other reactive gases also work) is necessary to limit grain growth. The substrate/coating temperature increases as the thickness of the coating increases, which can change the porosity of the coating as a function of thickness. U.S. Pat. No. 7,709,082 teaches a process where after some thickness of coating is deposited on an aluminum foil the process is stopped and repeated after a time period in order to produce discontinuous dendritic structures and thus enhance porosity and capacitance. The thickness of the aluminum foil substrates used in the prior art work is at least 25 microns or higher. A key limitation in using thinner foils is the strength of the foil at the high process temperature. When a coating of the order of a few microns is deposited on a thinner foil, the combined effect of the heat of condensation and to some extent the exothermic reaction in the presence of the oxygen impurity heats the foil substrate and weakens foil strength causing it to break, deform or melt. Therefore, there is a need for a low-temperature process that enables the formation of continuously growing, uninterrupted, dendritic structures dendritic structures, especially over large-area substrates.
Furthermore, in addition to the use of high-surface areas for capacitor applications, there is also a well-established need for utilizing such materials as well for absorbing radiation in the UV, Vis, NIR and FIR spectra, often combined with high thermal conductivity and super-hydrophobic and/or super-oleophobic properties. For example, missile-seeker sensors require reduction of stray light to improve image quality; cooled and uncooled IR detectors require IR absorbing materials with low outgassing and cryogenic compatibility; remote sensing of targets requires high/low emissivity coatings; mobile and stationary platforms need to passively dissipate heat from low-emissivity flat metal surfaces; IR optical systems, thermal cameras, conventional cameras, UV photolithographic optics, pyroelectric sensor arrays, space telescopes, endoscopes, etc, all require parts to suppress ghost images, trap light (sometimes down to a few photons) and passively dissipate heat. Therefore, there is a need for a surface modification process applicable to a wide range of materials that can combine several distinct functionalities, including broad spectrum radiation absorption, thermal transfer, color in the Vis spectrum and variable refractive index coatings, into a singular multifunctional surface treatment.